Various materials that are contained in the exhaust gas discharged from engines, particularly diesel engines, have become a cause of atmospheric pollution, and have caused various environmental problems. Particularly, particulate matter (PM), which is contained in exhaust gas, is said to be a factor that causes allergic symptoms such as asthma and hay fever.
In general, in a diesel engine for a vehicle, as an exhaust gas purification filter that traps particulate matter, a DPF (Diesel Particulate Filter) provided with a sealed type ceramic honeycomb structure (filter base body) has been used. In this honeycomb structure, both ends of cells (gas flow paths) of the ceramic honeycomb structure are sealed in a checkerboard pattern, and when exhaust gas passes through pores inside partition walls between these cells, the particulate matter is trapped (for example, see PTL 1 and PTL 2).
However, in such a DPF, since a pore size of the pores inside the partition walls is larger than a particle size of PM, particularly, in the early stages of use or in a state immediately after a regeneration treatment (clogging dissolution treatment of a filter through PM removal), there is a problem in that a the PM trapping rate is not sufficient. That is, in regard to DPF, since the trapping rate is improved after a PM layer has formed on a surface of each of the partition walls due to a certain amount of PM being trapped, there is a problem in that the trapping rate is low in the early stages of use or immediately after regeneration. In order to solve this problem, a structure provided with a micro-pore structure is disclosed, which has air permeability, traps PM, and in which aggregates of fine particles having fine gaps are connectedly provided in a surface of the partition wall or in the partition wall (PTL 3).
In addition, for the purpose of efficiently using a catalyst component in the DPF, a structure is disclosed in which a coat layer, which is formed from oxide particles such as alumina, is formed on a surface of a porous inorganic base material such as cordierite (PTL 4). The oxide particles have pores including inter-particulate gaps of 10 to 200 nm, and pore communication holes through which the pores communicate with each other and which include inter-particulate gaps of 10 nm or less. The catalyst component is supported in the pores of the coat layer (PTL 4).